1. Field of the Invention
The embodiments of the invention are directed generally to non-volatile NAND-type memory circuits.
2. Discussion of the Related Art
Non-volatile NAND-type semiconductor memory circuits are typically organized as strings of memory cells and fabricated on the surface of a semiconductor substrate. As density—as evaluated by the number of memory cells per unit surface area of the semiconductor substrate, for example—is an important design parameter, significant development efforts have been devoted to create memory circuits that pack multiple layers of memory cells in the “vertical” direction. In this detailed description, the term “horizontal” refers to any direction parallel to the surface of the semiconductor substrate, and the term “vertical” refers to any direction perpendicular to the surface of the semiconductor substrate. Similarly, all references to “top” and “bottom” are made considering the semiconductor substrate being at the bottom. However, a successful design requires not only a desirable density but also a fabrication process.